Editing Planetary nebula (section)

=== Central stars ===

The central stars of planetary nebulae are very hot.<ref name="Frankowskietal2009" /> Only when a star has exhausted most of its nuclear fuel can it collapse to a small size. Planetary nebulae are understood as a final stage of [[stellar evolution]]. Spectroscopic observations show that all planetary nebulae are expanding. This led to the idea that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life.<ref name=Kwok1/>